But the mammoth, historic city pool in Coral Gables – 820,000 gallons, about 40 times the size of a large home pool – is emptied and filled every night during the busy swimming season, when residents and tourists flock to the cool waters fed by the Biscayne aquifer.

Lifeguards Jose Vilar and Miles Charlton can attest to that. On a Sunday night in midsummer, they take fire hose and pressure cleaner to the bottom of the Venetian Pool, the only public pool listed on the National Register of Historic Places.

Carved out of a coral rock quarry, the "Venetian Casino" opened in 1924 with two waterfalls, a grotto and high diving platforms.

The high dives are no longer there. The accompanying coral rock buildings and covered walkway bear Mediterranean features, including barrel tile roofs. It is nestled amid plants of paradise – royal poinciana, bougainvillea, coconut palms – and once served as a popular music and lecture venue. Celebrated orator William Jennings Bryan spoke here. Paul Whiteman and his orchestra performed here. Swimming and movie stars Esther Williams (1944's Bathing Beauty) and Olympian Johnny Weissmuller (1930s Tarzan films) appeared here.

So inviting during the day, the sun reflecting off the water, Venetian Pool is a fascinating canyon of concrete and coral rock when it's empty at dusk. A few curious passersby press their faces against the wrought iron fence and watch Vilar and Charlton work.

"I love the place," says Vilar, 21, who grew up in nearby South Miami. "I learned to swim here." Charlton, 19, grew up in Coral Gables – and in the pool.

The two begin an elaborate behind-the-scenes operation after closing. Before they can clean the pool bottom, they wait 3.5 to 4 hours for the water to drain. Injection wells rapidly push the water down through the coral rock and limestone below, which filter it as it makes its way back into the aquifer.

"It's probably the best filtration that you can find," said Carolina Vester, pool supervisor.

That wasn't always the case. Until 1989, when the injection well system was installed, the water was just pumped out and wasted.

Vilar and Charlton push sand, leaves and other debris to a depression near the drain. They trap and remove the large bits of debris and pump the remaining particle-laden water out of the pool so that it doesn't go out with water bound for the aquifer.

When they finish, they turn on the pump, and fresh water from 50 to 75 feet underground gushes in through grates. It takes about 3.5 hours to fill.

The Venetian Pool uses the least amount of chlorine that regulations allow, one part per million gallons. It's easier on the eyes, the aquifer and the city budget, Vester notes. Because of the low dose of chlorine, the pool bans children younger than 3 – potty trained or not. Parents must be able to prove their child is at least 3.

Vester has been at the Venetian for eight years. She likes coming to work, she says, "because there's always something new happening every day."

Maintenance is constant in the nearly 90-year-old buildings, and to preserve the pool's character, every repair has to get approval from historical reviewers.

"Everything is a process in this facility, but it is a beautiful facility, and the fact that it is historic, it kind of maintains that original look from the '20s. And that's nice," Vester says.

Looking out over the roof from her office, Vester says that the barrel tile will have to be replaced soon. "Based on the historic aspect, they have to get exactly the same-looking barrel tile."

Of course, the barrel tile that has shielded the roof all these decades was handmade. The process would be too slow to replicate, and no doubt the "molds" would get tired.

With a small laugh, Vester tells the story: "Supposedly, they would put it over a female thigh to round the tile."